When comparing Dart vs F# (via FunScript), the Slant community recommends Dart for most people. In the question“What are the best languages that compile to JavaScript? ”Dart is ranked 13th while F# (via FunScript) is ranked 32nd. The most important reason people chose Dart is:

Dart is a single threaded programming language. So if any piece of code blocks the execution of the program, the program practically freezes. To avoid this Dart makes use of asynchronous operations which let your program run without getting blocked. This is done through Future objects.
A Future is an object which represent a means for getting a value at a certain point in the future. A function may invoke a Future and when that happens, two outcomes can be achieved:
1. The function is unable to return a value, so it queues up work to be done and returns an uncompleted Future object.
2. Or later when a value is available to be returned, the Future object completes with that value.

Pros

Pro

Great async language support

Dart is a single threaded programming language. So if any piece of code blocks the execution of the program, the program practically freezes. To avoid this Dart makes use of asynchronous operations which let your program run without getting blocked. This is done through Future objects.

A Future is an object which represent a means for getting a value at a certain point in the future. A function may invoke a Future and when that happens, two outcomes can be achieved:

The function is unable to return a value, so it queues up work to be done and returns an uncompleted Future object.

Or later when a value is available to be returned, the Future object completes with that value.

Pro

Great standard library

Dart includes a truly comprehensive core library, making it unnecessary to include disparate, external resources for basic functionalities Other than reducing the need to pull in various 3rd-party utilities this also ensures that all Dart code looks and feels the same.

Pro

No compile time in development

Dartium (Chromium derivative) is a browser with integrated Dart VM, which allows you to run and debug native Dart code during development for short edit-reload cycles. Only for testing on other browsers and deployment is transpiling to JS necessary.

Pro

Transpiled JavaScript code works on all browsers

In Dart many browser differences (subtle differences and also missing features) are abstracted away or polyfilled. When Dart is transpiled to JS the output works on all supported browsers. There is usually no need to load polyfills or to consider browser differences during development. No need for libraries like jQuery to make the same code work the same on all browsers.

Pro

Will be familiar to Java developers

The language will look familiar to Java developers, easing the learning curve.

And yet, while it's similar, it has some nice syntax facilities to avoid common boilerplate code found in Java. Code is terser, yet readable.

Pro

Easy prototyping

Dart has an optional type system which makes Dart a great language for prototyping. It encourages developers to gradually evolve their programs without worrying about types first.

Pro

Can compile to efficient machine code

Dart was designed to be as expressive as possible. Ahead-of-time compilers can compile Dart code to efficient machine code. This is especially important when deploying to mobile where you don't want (or can't) use a JIT.

Pro

Optional strong mode.

Strong mode applies a more restrictive type system to Dart to address its unsound, surprising behavior in certain cases.

Pro

AngularDart 2.0 support

Pro

Support of semi-coroutines (generators)

Generators, also known as semicoroutines, are also a generalization of subroutines.Generators are primarily used to simplify the control of iteration behavior of a loop, the yield statement in a generator passes a value back to a parent routine.

A generator is very similar to a function that returns an array, in that a generator has a certain number of values. But instead of building and returning an array that contains all the wanted values, a generator returns them one at a time, this saves memory and allows the caller function to start processing the first few values immediately.

Pro

The Dart to JavaScript compiler generates high quality source code

Dart to Javascript compiler (dart2js) generates very high quality source code with very high optimization. The output code is also very readable and easy to understand.

Pro

Crossplatform

Dart does not just compile to JavaScript, it also compiles to native code on mobile platforms like iOS and Android as demonstrated by flutter.io

Pro

Easier transition from other paradigms

Since F# is not a purely functional language, it lends itself to being more easily picked up by programmers that have experience with other paradigms.

Pro

Concise syntax

F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.

Pro

Runs on the CLR

Since F# runs on the Common Language Runtime or CLR, it has access to the entire .NET Framework, as well as libraries written in other .NET languages such as C#, VB.NET, and C++/CLI.

Cons

Con

Doesn’t generate consumable JavaScript at present

Dart builds to JavaScript but the entire app needs to be built to JavaScript at once for now (that may change in the future).

Con

Still many holes to fill while converting code to Javascript

While implementing callbacks, passing a type to any function that gets passed to Javascript causes the dart2js compiler to crash.

Con

Dart SDK does not provide standard (out of the box) way to access SQL-based databases on server side

This missing (but very popular) feature requires to use 3rd-party packages developed by the personal enthusiasts or very small groups of enthusiasts, which is not very convenient because they are all very fragmented in terms of content, the essence and capabilities.

Con

Small community, little momentum

Con

May be difficult to use some JavaScript libraries

Dart is much more than a programming language, it's a platform with its own standard libraries and tools. It's a major departure from JavaScript itself (even though it can compile to JS) that it's not possible to directly interact with JavaScript libraries in Dart. Instead, you have to use a special interop library which exposes the wrapped versions of the JavaScript objects that you are accessing.

While this enables Dart to sandbox JavaScript so that its problems do not leak into a Dart application, it also means that it may be cumbersome to use libraries which don't have a wrapper library available.

Con

Not really cross platform

Though the community sites are touting F# can be cross platform through use of Mono, the reality is that it is more of a hack to replay on larger mono applications in production.